G'day Pete. I'm no expert (disclaimer) but i have owned and operated both the motors in question here. Not a great deal of difference construction-wise, though that javelin is obviously an older motor, possibly approaching 30yrs?. My old man had one mounted on a 17ft pongrass half cab when i was a kid. I took it over when i got my boat license. it was a dog till it warmed up, but ran fine after that.
The 90 OMC (johnson/evinrude) of that era had a pretty good reputation. I have even heard the word 'bulletproof' used, but as usual, proper care and maintenance will dictate condition. That would also be an oil injected model? (VRO). Most are removed and premixed you will find. I've had them with both and not had problems either way, although the premix motor was noticably sand understandably smokier.
Both these motors had reasonably good reputations, but ultimately it will be how they were looked after that counts. Looks can be decieving - especially paint jobs! If you are not tech savvy, do yourself a favour and get any 2nd hand motor checked by someone who knows what they are doing and what they are looking for. It WILL work out cheaper inthe end.
Good luck, looking is half the fun!
Damo